The present invention relates to a feed rate control method and apparatus in a numerical control system, more specifically to a feed rate control method and apparatus at arcuate portions for feeding a tool at an appropriate feed rate in an arcuate path of movement for enabling high efficiency and highly accurate machining.
Numerically controlled machine tool technology using a built-in computer has advanced significantly because of rapid progress in computer technology and numerical control techniques. This technology is used for the production of complicated shapes of workpieces to be machined (such as parts of airplanes, automobiles, etc.) at high machining speeds. In order to process complicated curved surfaces at a high speed, there are increased numbers of arcuate portions to be machined. A frequently used technique is to move a tool in an arcuate manner even at straight (non-rounded) corners.
Disclosed in Japanese patent laid-open No. 72414/90 is a feed rate control method for machining at the maximum possible speed by moving the tool in a straight line at straight corners. This feed rate control method for numerical control comprises the steps of calculating the maximum tolerable rate of change in speed for each axis from the maximum tolerable torque by presetting the maximum feed rate Fmax and the maximum tolerable torque for each axis, calculating the change in speed for each axis at a specified path corner when an actual feed rate is set equal to the maximum feed rate, making a judgment if the change in speed for each axis is higher than time respective maximum tolerable change in speed, calculating the feed rate Fo to reduce the change in speed below the maximum tolerable change in speed if it is higher, and decelerating the feed rate at the corner from Fmax to Fo.
The above control method is effective in a case where the tool moves in straight lines at corners. However, the machining speed is limited in a case where a tool moves in an arcuate manner, because the acceleration changes very rapidly. Change in acceleration becomes the maximum at the transition from an arcuate machining to a linear machining. Such change in acceleration accompanies adverse movement or vibration (hereinafter referred to as mechanical shock) in the machine tool. This results in poor machining accuracy and is an obstacle to high speed machining because the tool cannot be fed at a high speed.
It is conventional in machining curved surfaces that the feed rate of time machine tool is decreased to a certain degree, as compared with the feed rate in a linear machining, whenever the machine tool reaches an arcuate portion. Deceleration in the feed rate remains relatively low if the radius of curvature of the arcuate portion is relatively large. However, the feed rate must be decreased significantly to avoid large mechanical shock if the radius is small. Deceleration in the feed rate at an arcuate portion is specified in the numerical control program and thus preset by the programmer. Such decelerated machining is not only limited to arcuate portions but also to completely circular machining.
The feed rate previously set in the NC program by the programmer is first read out to determine where to start deceleration so that the feed rate at the initial position of the machining block always remains below the speed specified for the block. A deceleration control apparatus a for automatically performing such deceleration are disclosed in U.S. Pat. No. 4894594 and European patent application publication number:299080. It can save time by the automatic deceleration control, but leaves the time consuming setting of the feed rate in the NC program.
As described above, it was typical in the conventional numerical control system to reduce the feed rate to a predetermined low rate at circular machining portions in a linear-curved surface-linear machining step. Such reduction in feed rate depends primarily on the experience of each programmer.
However, acceleration changes significantly at the transition between the circular and the linear machining portions. In the conventional approach to set the speed by the programmer's experiences or various experiments, it is almost impossible to set the optimum feed rate at arcuate machining portions. It is normally set to substantially lower rate for safety purpose or to provide a margin. This is an obstacle to a high speed machining. Additionally, tools vary in size, thereby varying the distance or offset between the workpiece to be machined and the center of the tool. As the offset varies, the radius of an arcuate machining also varies and thus varying the optimum feed rate for such arcuate machining. This makes it more difficult to set the optimum feed rate.
It may be possible to obtain the optimum feed rate through experiments using the actual workpiece to be machined and the actual tool. However, the number of arcuate machining portions increases progressively as the complexity of the machining shape increases. This approach is also very difficult if you take the required tools into consideration.
In high speed machining, fluctuation phenomena of the machine due to sudden change in acceleration cause different machining accuracies depending whether the tool travels through a joint between straight and arcuate portions or a completely circular portion. That is, accuracy is lower at arcuate portions as compared with completely circular portions.
Nevertheless, it was the conventional to decelerate the feed rate at a constant factor of the automatic speed control system at both arcuate and completely circular portions. Accordingly, the overall machining speed cannot be increased, despite the fact that no machining accuracy problem is caused at completely circular portions if machined at the constant of the automatic speed control system for arcuate portions.
It is therefore an object of the present invention to provide a method and an apparatus for feed rate control at arcuate portions capable of minimizing mechanical shock to the tools machining curved portions, and achieving higher machining speed and accuracy by optimum setting of the feed rate at arcuate portions.
It is another object of the present invention to provide a method and an apparatus for achieving feed rate control at arcuate portions including both arcuate and completely circular machining portions capable of increasing the overall machining speed by setting a higher feed rate at completely circular machining portions than that at arcuate machining portions while maintaining a predetermined accuracy.